Method of characterising a tuneable laser

ABSTRACT

A method of evaluating a tuneable laser and determining suitable laser operation points. Part of the light emitted by the laser is led to a Fabry-Perot filter and to a pair of light detectors to measure the power of the laser and deliver a corresponding output signal representative of the wavelength of the detected light. Injected currents are swept through the laser tuning sections to pass through different current combinations, and the ratio between the detector signals is measured during the sweep of the reflector current in both directions. The control combination for the tuning currents is stored when the ratio between the detector output signals lies within a predetermined range, signifying that the emitted light lies within one of a number of wavelengths given by the Fabry-Perot filter, and the ratio lies within the predetermined range for a given reflector current in both sweep directions of the reflector current.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of enabling a tuneable laser to be characterized quickly.

[0003] The method can be applied for evaluating and selecting lasers with respect to emitted wavelength and to find good operation points systematically.

[0004] 2. Description of the Related Art

[0005] Tuneable semiconductor lasers have a number of different sections through which current is injected, typically three or four such sections. The wavelength, power and mode purity of the lasers can be controlled by adjusting the current injected into the various sections. Mode purity implies that the laser is at an operation point, i.e., at a distance from a combination of the drive currents where so-called mode jumps take place, and where lasering is stable and side mode suppression is high.

[0006] In the case of telecommunications applications, it is necessary that the laser is able to retain its wavelength to a very high degree of accuracy and over very long periods of time, after having set the drive currents and the temperature. A typical accuracy in this respect is 0.1 nanometer and a typical time period is 20 years.

[0007] In order to be able to control the laser, it is necessary to map the behavior of the laser as a function of the various drive currents. This is necessary prior to using the laser after its manufacture.

[0008] Mapping of the behavior of a laser is normally effected by connecting the laser to different measuring instruments and then varying the drive currents systematically. Such instruments are normally power meters, optical spectrum analyzers for measuring wavelength and sidemode suppression, and line width measuring devices. This laser measuring process enables all of these parameters to be fully mapped as a function of all different drive currents.

[0009] One problem is that lasers exhibit hysteresis. As a result of the hysteresis, the laser will deliver different output signals in the form of power and wavelength in respect of a given drive current set-up, i.e., with respect to a given operation point, depending on the path through which the laser has passed with respect to the change in said drive currents, in order to arrive at the working point in question. Thus, this means that a given drive current set-up will not unequivocally give the expected wavelength or power.

[0010] In the case of a tuneable laser, the wavelength of the emitted light is determined mainly by the current or voltage across the tuning sections. The power emitted is controlled by current to the gain section of the laser or by the voltage across said section.

[0011] When characterizing a laser, all of the possible control combinations afforded by the tuning sections, or a subset of said sections, are investigated. During the characterizing process, the emitted light is studied with respect to wavelength and sidemode suppression and controlling the gain section with regard to power adjustment.

[0012] The enormous number of possible control combinations, typically tens of billions, of which fewer than a hundred will be selected, makes total mapping of the laser impossible in view of the large amount of data generated.

SUMMARY OF THE INVENTION

[0013] The present invention solves this problem and provides a method of quickly sorting away control combinations that do not result in correct wavelengths.

[0014] The present invention also relates to a method of evaluating a tuneable laser and determining suitable laser operation points for a laser that includes two or more tuneable sections, in which injected current can be varied and of which at least one is a reflector section and one is a phase section. The method includes leading part of the light emitted by the laser to an arrangement that includes a Fabry-Perot filter and a first and a second detector, said detectors being adapted to measure the 0power of the light and to deliver a corresponding detector signal. The detectors are arranged relative to the Fabry-Perot filter such that the data signals will contain information relating at least to the wavelength of the detected light. The currents are swept through the tuning sections so as to pass through different current combinations, and the ratio between the two detector signals during said sweeps is measured. The reflector current in the inner sweep variable is swept in one direction and then in the opposite direction back to its start value. The control combination for the tuning currents is stored when the ratio between the detector signals lies within a predetermined range that indicates that the light emitted lies within one of a number of wavelengths given by the Fabry-Perot filter, and when the ratio lies within said predetermined range for a given reflector current in both sweep directions of said current.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The invention will now be described in more detail with reference to exemplifying embodiments thereof and also with reference to the accompanying drawings, in which

[0016]FIG. 1 is a perspective, partially cut-away view of a DBR laser;

[0017]FIG. 2 is a sectional view of a tuneable Grating Coupled Sampled Reflector (GCSR) laser;

[0018]FIG. 3 is a sectional view of a Sampled Grating DBR laser; and

[0019]FIG. 4 is a schematic block diagram illustrating an arrangement which is used in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Shown in FIG. 1 is a DBR laser which includes three sections, namely a Bragg reflector 1, a phase section 2 and a gain section 3. Each section is controlled by injecting current into respective sections through respective electric conductors 4, 5, 6.

[0021]FIG. 2 is a sectional view of a tuneable Grating Coupled Sampled Reflector (GCSR) laser. Such a laser includes four sections, i.e., a Bragg reflector 7, a phase section 8, a coupler 9 and a gain section 10. Each of the sections is controlled by injecting current into the respective sections.

[0022]FIG. 3 is a sectional view of a Sampled Grating DBR laser that also includes four sections 11, 12, 13, 14, of which sections 11 and 14 are Bragg reflectors, section 13 is the phase section, and section 12 is the gain section.

[0023] These three laser types are common, although other types of lasers exist.

[0024] Although the invention is described below essentially with reference to a GCSR laser according to FIG. 2, it will be understood that the invention is not restricted to any particular type of tuneable semiconductor laser, but can be applied correspondingly with tuneable lasers other than those illustrated by way of example in the drawings.

[0025] The present invention relates to a method of evaluating tuneable lasers and determining suitable laser operation points. The laser may thus contain two or more tuneable sections in which injected current can be varied in a known manner. The laser is of the kind which includes at least one reflector section and one phase section.

[0026]FIG. 4 is a block diagram which illustrates an arrangement used in accordance with the present invention. The reference numeral 15 identifies a GCSR laser, while the reference numeral 16 identifies current generators for injecting current into the reflector section, the phase section and the coupler section respectively of said laser, through respective conductors 17, 18 and 19. The power of the laser is controlled to its gain section by means of a power regulating circuit 20, via a conductor 21.

[0027] The laser emits light from the front mirror to a light conductor 23, for instance a light-conducting fiber, via a lens pack 22. This light conductor leads the light to a light splitter or divider 26 which switches part of the light to another light conductor 24. The remainder of the light is led further in the light conductor 25. The light splitter 26 switches, e.g., 10%, of the light from the conductor 23 to the conductor 24.

[0028] The light conductor 24 leads the light to a second light splitter or divider 27, which functions to divide the light equally between two light conductors 28, 29. A lens 30 and a lens 31 are disposed at respective ends of the light conductors. A Fabry-Perot filter 32 is provided in the beam path downstream of the lens 30. The filter 32 is well known and will not therefore be described in more detail in this document. Fabry-Perot filters can be designed to exhibit a certain light transmission solely for certain wavelengths, normally wavelengths that are multiples of a given wavelength. The Fabry-Perot filter exhibits a deviating lower or higher transmission at other wavelengths.

[0029] A first detector 33 is provided downstream of the lens 31, and a second detector 34 is provided downstream of the Fabry-Perot filter. The detectors 33, 34 function to measure the power of the light and to deliver a corresponding detector signal to an A/D converter 37, via a respective amplifier 35, 36.

[0030] The A/D converter 37, the power regulating circuit 20 and the current generators 16 are all connected to a microprocessor 39 via a data bus 38. The microprocessor is adapted to control the current generators and the power regulating circuit in a desired and a well known manner, in response to the signals from the A/D converter 37 and the power regulating circuit 20.

[0031] According to the invention, part of the forwardly emitted light is thus conducted to the first detector 33 and also to the second detector 34, via the Fabry-Perot filter 32.

[0032] According to the invention, the currents are swept through the tuning sections 18, 19, 21 such as to pass through different current combinations. The ratio between the two detector signals I1 and I2 is measured during said sweeps.

[0033] When sweeping the currents through the tuning sections, the reflector current is the inner sweep variable. It is meant by this that the reflector current is swept for different combinations of other tuning currents while holding said other currents constant. The reflector current is swept first in one direction and then in the opposite direction, back to its start value. For instance, the reflector current is swept from a zero value and up to its maximum value and then down to zero again.

[0034] By current control in the present document is meant that the current through the sections is controlled by current generators or, alternatively, by controlling the voltage across the sections.

[0035] In the case of the FIG. 4 embodiment, the first detector 33, the second detector 34 and the Fabry-Perot filter 32 are placed in the proximity of the front mirror of the laser. Alternatively, the components may equally as well be placed in the proximity of the rear mirror of the laser, in which case light emitted from the rear mirror of said laser is used to determine the wavelength.

[0036] The Fabry-Perot filter and the first and the second detector may be arranged relative to one another in a manner different from that shown in FIG. 4, so as to detect at least wavelengths. The first and the second detector may be arranged to measure light transmitted through the Fabry-Perot filter and/or light reflected towards the Fabry-Perot filter, such as to detect wavelengths.

[0037] The hysteresis effect exhibited by lasers causes the power output of the laser in respect of certain reflector currents, with otherwise constant conditions, to be different due to the reflector current having taken its existing value by virtue of the reflector current having increased to said value or having decreased from a higher value. The wavelength is also influenced by the hysteresis effect. Such operation points as those which lie in the regions of hysteresis with respect to the reflector current, or with respect to other tuning currents for those sections that exhibit hysteresis, are non-preferred operation points for a laser in operation.

[0038] Communication lasers are adapted to operate at certain given wavelengths that are included in a so-called channel plane, where each channel corresponds to a well defined wavelength. According to the invention, the Fabry-Perot filter 32 is adapted to have a certain given transmission for each wavelength included in the channel plane.

[0039] When the ratio between the detector signals I1/I2 from the detectors 34, 33 lies within a predetermined range, implying that the emitted light lies within one of a number of wavelengths given by the Fabry-Perot filter, and said ratio I1/I2 lies within said range for a given reflector current in both sweep directions of the reflector current, the control combination for the tuning currents is stored, in accordance with the invention.

[0040] This range is given by the permitted channel width in the channel plane.

[0041] These control combinations thus fulfill the criteria that will give desired wavelengths and not result in any hysteresis effect.

[0042] In certain cases it is preferred that one or more other tuneable currents to sections that exhibit a hysteresis effect, excluding the reflector current, are swept so as to determine whether or not hysteresis occurs at a contemplated operation point.

[0043] According to one preferred embodiment, the signal 12 is delivered from the first detector 33 to the power regulating circuit 20. The regulating circuit is adapted to control the laser so that said laser will emit light at a constant power. This enables the ratio I1/I2 to be followed very easily in determining possible operation points.

[0044] According to another preferred embodiment of the invention, a monitor diode 40 is placed on the side of the laser opposite to that side on which the first and the second detectors are placed, said monitor diode being caused to measure the light emitted by the laser. The detected light measurement signal is led through an amplifier 41 to an A/D converter 42, whose output signal is delivered to the microprocessor 39. In this embodiment, one or more of the tuneable currents is chosen so as to minimize the ratio between the power of the rearwardly emitted light and the power of the forwardly emitted light, thereby enabling an optimum operation point for a channel to be selected from said possible operation points.

[0045] The monitor diode 40 is placed adjacent the rear mirror of the laser in the FIG. 4 embodiment.

[0046] It is highly preferred with a number of the possible operation points taken out to measure the wavelength emitted by the laser until an operation point has been obtained with each desired wavelength, wherewith the control combination for each operation point is stored. Thus, one control combination for each channel in the channel plane will be stored in the memory of the microprocessor.

[0047] It will be evident from the foregoing that the use of a Fabry-Perot filter enables all those control combinations that do not fulfil the criterion that the ratio between the currents I1/I2 shall lie within a certain given range to be sorted out. Moreover, it is sufficient for communications purposes to identify one control combination for each wavelength in the channel plane that lies in a region in which the laser exhibits no hysteresis.

[0048] The present invention thus solves the problem mentioned in the introduction.

[0049] Although different embodiments have been described, and therewith in respect of a GCSR laser, it will be obvious that the structural design of the described arrangement can be varied while achieving the same result. The invention can also be applied to lasers of a type other than GCSR lasers.

[0050] It will therefore be understood that the present invention is not restricted to the aforedescribed and illustrated exemplifying embodiments thereof and that variations can be made within the scope of the following claims. 

1-6. (canceled)
 7. A tunable laser system, comprising: a semiconductor laser having at least three laser sections, the at least three laser sections being individually addressable with respective control currents, the at least three laser sections comprising a gain section and at least two tuning sections, the at least two tuning sections comprising at least a phase section and a first reflector section; and a laser control unit coupled to apply respective control currents to the at least three laser sections, the current control unit storing information describing different combinations of the respective control currents, the different combinations of the respective control currents being associated with different operation points of the laser.
 8. The laser system as recited in claim 7, wherein the at least two tuning sections further comprise a second reflector section.
 9. The laser system as recited in claim 7, wherein the at least two tuning sections further comprise a coupler section.
 10. The laser system as recited in claim 7, wherein the laser control unit comprises a current generator unit that supplies the control currents applied to the at least two tuning sections.
 11. The laser system as recited in claim 7, further comprising a power detector optically coupled to receive an optical output from the laser, and wherein the laser control unit further comprises a power regulating unit adapted to direct a current to the gain section so as to emit light at constant power.
 12. The laser system as recited in claim 7, further comprising a frequency-sensitive detector unit coupled to the control unit, the frequency sensitive detector unit optically coupled to receive light output from the laser and coupled to provide a frequency signal to the laser control unit, frequency signal being indicative of the frequency of the light output from the laser.
 13. The laser system as recited in claim 12, wherein the frequency sensitive detector unit comprises an optical filter having a frequency-dependent filter characteristic and a light detector disposed to detect an optical signal modulated by the frequency-dependent filter characteristic of the optical filter.
 14. The laser system as recited in claim 13, wherein the optical filter is a Fabry-Perot filter.
 15. The laser system as recited in claim 7, wherein the laser control unit further comprises a microprocessor that controls selection of the different combinations of the respective control currents applied to the laser.
 16. A method of operating a semiconductor laser having at least two tuning sections receiving individually addressable control currents, the method comprising: storing combinations of control currents, each combination of control current being associated with a predetermined laser operating wavelength; and applying a particular combination of control currents to the laser, the particular combination of control currents applied to the laser being associated with a desired laser operating wavelength.
 17. The method as recited in claim 16, further comprising detecting an optical signal from the laser indicative of power output from the laser and controlling the power output from the laser to be stable based on the detected optical signal.
 18. The method as recited in claim 16, wherein applying the particular control currents to the laser comprises applying a control current to at least a first reflector section and a phase section of the laser.
 19. The method as recited in claim 18, wherein applying the particular control currents to the laser further comprises applying a control current to at least a second reflector section of the laser.
 20. The method as recited in claim 18, wherein applying the particular control currents to the laser further comprises applying a control current to a coupler section of the laser.
 21. The method as recited in claim 16, further comprising passing light from the laser to a frequency-sensitive detector and producing a frequency signal.
 22. The laser system as recited in claim 12, wherein passing the light from the laser to a frequency-sensitive detector comprises passing the light to a filter having a frequency dependent filter characteristic and detecting light modulated by the filter. 